Simple bit counter for 32bit integers

main.c

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>

int
count_bits(int n)
{
	int l, c;
	l = 0;
	c = 0;
	while (l < 32) {
		if (n & (1 << l++)) {
			c++;
		}
	}

	return c;
}

int main(int argc, char *argv[])
{
	int n, c;

	if (argc != 2) {
		fprintf(stderr, "Usage: %s NUMBER\n", *argv);
		return 1;
	}

	n = atoi(*(argv+1));
	if (n < 0) {
		fprintf(stderr,
			"Number too large. Must be lower than %ld\n",
			(long) INT_MAX + 1);
		return 1;
	}
	c = count_bits(n);
	printf("%d: %d\n", n, c);

	return 0;
}